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Wall Calculator Forces appear to be inaccurate compared to Hand Calcs on an Airfoil

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Message 1 of 2
ryandhatton
217 Views, 1 Reply

Wall Calculator Forces appear to be inaccurate compared to Hand Calcs on an Airfoil

Hey all,

 

I've been honing my Autodesk CFD skills over these last few months and have run hundreds of simulations at this point. I'm still struggling to get accurate numbers compared to my theoretical expected results when doing external aerodynamic simulations.

 

I am attempting to validate the results on an FX74 Airfoil in Autodesk CFD. The wing has a chord length of 225 mm and a span of 1600 mm. AirfoilTools provided the below coefficients of lift. 

FX74 Lift Coefficients.PNG

Based on this info, I should be getting around 119 lbf of downforce (of Lift) at 45 m/s and 0 Degrees AOA.

 

I've run through the simulation several times. The first run was a 900 iteration single run and I got around 93 lbf which is close-ish. I ran it again with Mesh Adaptation and Y+ turned on and got 83 lbf which is trending in the wrong direction. I ran a 3rd simulation with a Mesh Refinement Region and 750 iterations and it converged with ADV5 and Tight convergence at 355 iterations. This 3rd run I got the following:

FX74 Wall Calculator.PNG 

FX74 Convergence Plot.PNG

 

It seems as though the more refined my mesh gets, the further away I get from the expected results of the simulation.

 

I can attach my support file but here are some tidbits from my sims that I think would be helpful for you experts to help me figure this out and keep my sanity!

 

Here is my mesh in the 3rd run with the refinement region:

FX74 Mesh.PNGBirds Eye FX74 Mesh.PNGSurface Mesh on the Wing.PNGMesh Region Settings FX74.PNG

 

 

Here are my mesh settings:

Mesh Settings FX74.PNGMesh Region Settings FX74.PNG

 

Boundary Conditions of the enclosure

Inlet --> Velocity 45 m/s

Outlet --> Pressure 0 Gage

Slip/Symmetry on all Walls and Floor/Ceiling

 

Initial Conditions 

Inlet --> Velocity 45 m/s

Outlet --> Pressure 0 Gage

 

Here are my Flags I have turned on:

FX74 Flags.PNG

 

Solution Control

-Turbulent

-SST K-Omega (Hellsten) --> I used this since it has the RC flags built into the mix and seems to give me more consistent results on an airfoil sim.

-ADV5 with Tight Convergence

 

 

Any of the gurus see any glaring mistakes? I've watched every Autodesk Provided Airfoil Lecture I can find on the forums and Youtube. It's been helpful for learning the setup and flags but I still can't seem to get reasonable results. Is this still somehow a mesh issue? Is my mesh not refined enough? Every time I refine it more, the wall calculator numbers get worse which is odd. 

 

Thanks community for your help!!!

 

 

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Message 2 of 2
sancho78rus
in reply to: ryandhatton

I am just now solving this problem. Here's what I found about this: https://damassets.autodesk.net/content/dam/autodesk/www/pdfs/CFD_External_Flow_Validation_for_Airfoi...

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